1. Field of the Invention
The present invention relates generally to RF devices, and particularly to relatively high power RF devices with improved thermal characteristics.
2. Technical Background
Nowadays, the primary use of some mobile phones (e.g., smartphones) is data communications rather than voice communications. More and more of the public is using their mobile telephones for Internet access, social media access and for downloading videos and pictures from both of these sources. Thus, the demand for mobile data services is growing at an exponential rate. This necessarily means that the bandwidth provided by the telecommunications infrastructure (as well as the mobile telephone handset itself) must continue to grow if it is to keep pace with the demand. In order to accomplish this, new mobile telephone handsets and new base station equipment must be configured to operate over multiple frequency bands and communications systems (e.g., Wi-Fi, 3G, 4G, Bluetooth, etc.). Stated differently, new mobile telephones and base stations must provide more functionality in the same (or less) space. Because radio frequency (RF) devices such as power amplifiers, couplers, baluns, etc., are integral parts of every wireless device or system, they must also do more in a smaller footprint. Accordingly, size reduction, improved heat dissipation, power handling, and cost reduction are important considerations for today's RF component designs.
Referring to FIG. 1, a sectional view of a conventional softboard RF device 1 is shown. The RF device 1 includes three softboard layers (1, 2, and 3) that are laminated together to form an integrated device. Before lamination is performed, conductive traces 4 are disposed on the middle softboard layer 2 by way of conventional photolithographic methods. One of the advantages of using all-softboard related devices relates to their excellent RF performance. One reason for this is that softboard devices use photolithographic processes for circuit trace fabrication; photolithography produces excellent conductive traces (i.e., high purity copper traces that have an excellent shape) that exhibit low conductor resistivity (e.g., Cu—1.7 μΩ-cm; Ag—1.55 μΩ-cm; and Au—2.2 μΩ-cm). Moreover, softboard devices are easily configured to match the coefficient of thermal expansion (CTE) of the SMT carrier printed circuit board (PCB). As a result, the reliability of softboard devices is typically excellent. Finally, softboard devices are amenable to being manufactured in large batches, which typically results in lower costs.
On the other hand, the maximum power handling abilities of softboard based RF circuits is relatively low because they are characterized by relatively poor thermal resistance and Maximum Operating Temperature (MOT). Power Handing is a function of thermal conductivity, electrical performance, circuit geometry, and part size. MOT is a function of material properties. Typical FR4 has a MOT of 130° C. Teflon based PCB materials have a MOT up to 280° C. Polyamides have a MOT>200° C. Copper has an MOT of 130° C. in air. The material with the lowest MOT sets the limit for the entire system. Standard softboard dielectrics have a relatively low thermal conductivity of about 0.25 W/mK up to 1 W/mK, with doped softboard dielectrics reaching up to about 3 W/mK.
Referring to the heat distribution pattern 5 shown in FIG. 1, the amount of heat that is laterally conducted by softboard type devices is rather limited. Those skilled in the art assume that the thermal energy generated by all-softboard devices dissipates at about a 45° angle through the softboard dielectric materials. Thus, the thermal distribution pattern 5 is limited to a small region on each major surface of the device 1. Accordingly, one drawback to this approach is that the thermal energy is dissipated over a rather small surface area, and as a result, the thermal energy builds up over time and becomes problematic. In other words, the power handling of the device is limited by its heat dissipation capabilities. All-softboard devices typically exhibit a low maximum continuous operating temperature (e.g., about 120° C. to 200° C. typical). The glass transition temperature (Tg) is also relatively low (e.g., 75° C. to 150° C. being typical values for standard PCB materials).
Referring to FIG. 2, a top view of the conventional softboard RF device depicted in FIG. 1 is shown. This view represents a typical configuration that provides three edge vias 6 on each side thereof, with one edge via 6 disposed on each end. Using the dimensions in the illustration, and assuming 1 mil thick copper, the surface area provided by each via is about 0.05 mm2. The combined area (for three vias) is about 0.149 mm2. Even if one were able to direct the thermal energy to the edge of the device, there is only a small surface area available for heat dissipation.
Instead of using softboard dielectric layers, some manufacturers produce RF circuits (e.g., couplers) using low temperature co-fired ceramic (LTCC) or (high temperature co-fired ceramic) HTCC ceramic technology. Some of the advantages of ceramic devices include: high thermal conductivity (30 W/m° C. to 170 W/m° C. typical); low thickness variations (when lapped +/−0.0005 in typical); high maximum continuous operating temperatures (500° C. to 2000° C. typical); and a high glass transition temperature (Tg). On the other hand, the maximum power handling of ceramic RF devices is limited by solder joint failure (Reflow or cracked solder joint) at the device-to-carrier PCB interface (solder joint reflow often leads to a failed solder joint). For the solder reflow failure mode, the all-ceramic RF device can become so hot that the solder reflows and the device separates from the carrier PCB. Stated differently, the high thermal conductivity of conventional ceramic devices can lead to device failure if the power level is not limited. The cracked solder joint failure mode is typically due to the CTE mismatch between the all-ceramic RF device and the softboard carrier PCB (Ceramic exhibits a CTE of about 5 ppm/T typical whereas the carrier PCB exhibits a CTE of about 15 to 25 ppm/° C. typical in x/y axis).
In addition, all-ceramic RF devices have a relatively low RF performance (vis à vis softboard devices) due to the conductor material and conductor shape tolerance. Thus, RF circuits based on either all-softboard or all-ceramic constructions have inherent features which limit their maximum high power rating and reliability. Moreover, when an RF device is used in a surface mount technology (SMT) assembly, the device is mounted on a carrier PCB and the thermal path is further degraded by the carrier PCB.
Referring to FIG. 3, a cross-sectional view of a conventional LTCC or HTCC (all-ceramic) device 1 is shown. The ceramic device 1 includes three ceramic layers (2, 3 and 4) with traces 5 formed on the center ceramic layer 3. In this view, the cross-sectional shape of the conductor 5 is of interest. A conventional all-ceramic RF device 1 typically employs a process whereby the circuit traces are screen printed. As a result, the conductive traces 5 exhibit high resistivity due to porosity and impurities. The high resistivity of the conductors 5 is due to their being made from an Ag or a W (5.7 microohm-cm) paste. Moreover, the cross-sectional shape is not conducive to high RF performance. Accordingly, the performance of these RF devices is poor relative to softboard devices. Finally, the size of the footprint of a panel of ceramic devices (before singulation) is relatively small (e.g., about 4″×4″ to 6″×6″) as compared to softboard panels (which may be 12″×18″ to 18″×24″ before singulation).
Referring to FIGS. 4A and 4B, cross-sectional views of conventional surface mount technology (SMT) power amplifiers 1000 are shown. Often, conventional power amplifiers are fabricated using SMT plastic over-molded transistors because these components offer the lowest cost option to the manufacturer. As shown in FIG. 4A, a transistor die 1002 is soldered or epoxied (20) to a copper slug 1008 for heat dissipation purposes. The RF input contact pad 1012 and the RF output contact 1014 are connected to PCBs 1010 by leads 1006. The PCBs or ceramic boards 1010 are connected to the transistor 1002 via leads 1004. The entire device 1000 is covered by a plastic over-mold 1100. The copper slug 1008 is then mounted on a carrier PCB 22 (not shown) so that the maximum operating temperature of the die 1002 and the plastic over-mold 1100 is not exceeded. FIG. 4B is another conventional power amplifier 1000 example, and includes flip chip amplifier 1002 mounted on PCB (or ceramic board) 1010. The pads or solder balls of the flip chip 1002 are connected to corresponding pads of the PCB 1010 by solder connections 1016. The PCB (or ceramic board) 1010 is mounted on copper slug 1008 which functions as a heat sink. This arrangement includes leads 1004 which are used to connect the board 1010 to the I/O pads 1012, 1014.
FIG. 5 is a plan view of a circuit configuration 1200 that includes the conventional power amplifier 1000 depicted in FIG. 4. Note that the transistors (1002) have an input and output impedance that is significantly lower than the 50 Ohm system impedance, and therefore, a matching network is needed. As a result, the conventional SMT plastic over-molded transistor 1000 shown in FIG. 5 requires both an input matching network 1014 and an output matching network 1016. As a result, the device configuration 1200 occupies too much PCB “real estate.”
What is needed, therefore, is an improved RF device that solves the coefficient of thermal expansion (CTE) mismatches and RF performance issues related to ceramic based circuits, while simultaneously solving the heat dissipation issues related to softboard based circuits. An RF device is needed that efficiently distributes heat laterally throughout substantially the entire center substrate layer for improved thermal performance. An RF device is needed that has a PCB interface layer that is substantially matched to the CTE of the carrier PCB (i.e., has a relatively low Modulus of Elasticity) to prevent solder joint failure. Finally, an RF device is needed that has all of the above stated improvements in a small compact package that occupies a relatively small footprint on the carrier PCB.